The present invention concerns methods and circuits for detecting errors in digital signals, of the type so encoded that the running digital sum of the digital signal monitored is confined between predetermined lower and upper limits, i.e., if the digital signal at the location monitored is free of error. With such methods and systems, if the running digital sum falls below the lower limit value or exceeds the upper limit value, this triggers an indication of faulty signal transmission.
In general, imposition of the criterion that the running digital sum (hereafter, RDS) of a digital signal be continuously confined within such limits requires resort to redundancy techniques because, in for example the case of a bit stream constituting transmitted information, the number of "0" bits versus the number of "1" bits is, in the absence of such redundancy, not controllable but instead dictated by the information to be transmitted.
Resort to a redundancy technique generally involves an increase in the number of logic levels to be exhibited by the transmitted digital signal. Alternatively, or in addition thereto, in order that the rate of transmission of actual information not be decreased, it is necessary to resort to an increase in the rate of transmission of the constituent digits of the redundancy-exhibiting digital signal, i.e., because of the word-length increase resulting from the redundant bits or digits.
An example of the first alternative--i.e., an increase in the number of logic levels employed--is to be found in the alternate mark inversion (AMI) technique. Use is made of a pseudoternary code, according to which every second "1" of the original bit stream is converted to a "-1" in the corresponding ternary signal. The rate of transmission of ternary bits can be the same as the bit transmission rate of the original binary signal. Inherently with such a technique, the RDS of the digital signal can only assume the three values -1, 0 and +1, i.e., assuming that the signal is being transmitted without error.
Both an increase in the number of logic levels employed, and an increase in the rate of digit transmission, are utilized in conjunction when resorting to redundancy techniques in the case of ternary block codes. For example, when employing a 4B/3T-code, the original binary signal is constituted by words of 4 bits each, and the corresponding ternary signal is constituted by words of three digits each, each digit being capable of assuming any one of three different logic levels. Accordingly, the digit transmission rate of the corresponding ternary signal can be reduced to three-fourths the value needed for the original binary signal, i.e., if the word transmission rate is to remain the same. If, now, resort is to be had to a redundancy technique, for the sake of a limited RDS, the reduced digit transmission rate made possible by the use of 4B/3T-code must be increased back to its original value.
The second of the two alternatives mentioned above--i.e., increase of the digit transmission rate employed--is used alone in the case of binary block codes. For example, when employing a 5B/6B-code, each word of the original redundancy-free signal is constituted by five bits, but for transmission purposes each original redundancy-free word is represented by a 6-bit word. This necessitates a 20% increase of the bit transmission rate employed, if the word transmission rate is not to be decreased. As persons skilled in the art will understand, as each original 5-bit word comes along for encoding into 6-bit code, the encoding scheme in accordance with which 6-bit words are assigned to represent the original 5-bit words can be so established that the RDS of the successive bits of the stream of 6-bit words can only assume seven different values. If each "1" bit in the stream of 6-bit words is assigned the value +1, and each "0" bit the value +1, then the encoding scheme in accordance with which 6-bit words are assigned to represent the original 5-bit words can be so established that the RDS of these values (+1 and -1) of the constituent bits of the stream of 6-bit words can only assume the values -3, -2, -1, 0, 1, 2 and 3, i.e., absent error in transmission. Accordingly, if at any time the absolute magnitude of the RDS is greater than 3, this indicates transmission error, e.g., interference, faulty operation of a repeater, or the like.
In this way, it becomes possible to monitor the RDS and accordingly the bit error rate at each repeater station of the communications path along which the digital signal is being transmitted, and furthermore, in per se conventional manner, to transmit this information concerning bit error rate to the ultimate destination of the transmitted digital signal.
This form of error detection is of particular importance when very high digit transmission rates are involved, because it does not require interruption of transmission of actual information, and accordingly loss of large amounts of information or loss of the possibility of its transmission, merely for the purpose of transmitting test signals or the like.
When this form of error detection is employed, use is generally made of forwards-backwards digital counters, for example operative for counting forwards by one in response to each "1" bit of a binary digital signal and counting backwards by one in response to each "0" bit to generate the requisite RDS. As soon as the counter's count exceeds a preset upper limit or falls below a preset lower limit, this triggers a bit-error indication. Such a technique is disclosed, for example, in Federal Republic of Germany published patent application DE-OS No. 20 30 763.
The digital circuitry needed for such exclusively digital error-detecting techniques can be of considerable cost. A further disadvantage of exclusively digital techniques relates to relatively high power consumption. For example, in the case of high digit transmission rates, e.g., 41 Mbits/sec., the error-recognition circuitry must in general be constituted by Schottky-TTL components and the power consumption of such an error-detection circuit can amount to almost 1 watt. Such a power-consumption level is very problematic where the repeater stations along the transmission path are not provided with local power supplies but instead with transmitted power from remote sources.